The Thyroid Stimulating Hormone (TSH) assay, identified by CPT code 84443, is a primary diagnostic laboratory procedure used to quantify the level of thyrotropin in a patient's blood. TSH is a glycoprotein hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland. Its primary physiological role is to regulate the endocrine function of the thyroid gland. This process is governed by a complex negative feedback mechanism known as the hypothalamic-pituitary-thyroid (HPT) axis. When circulating levels of thyroid hormones—specifically thyroxine (T4) and triiodothyronine (T3)—drop, the hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to release TSH. TSH then stimulates the thyroid gland to produce more hormones. Conversely, when T3 and T4 levels are elevated, TSH production is inhibited. Because of this sensitive relationship, measuring TSH is considered the most effective first-line screening tool for thyroid dysfunction. An elevated TSH concentration typically indicates primary hypothyroidism, where the thyroid gland is failing to produce adequate hormones despite increased pituitary stimulation. A low or suppressed TSH level usually suggests hyperthyroidism, where the thyroid is overproducing hormones, signaling the pituitary to minimize TSH output. This assay is also vital for monitoring patients on thyroid replacement therapy, evaluating pituitary disorders, and screening newborns for congenital hypothyroidism. Modern 84443 assays typically utilize high-sensitivity immunoassay techniques, such as chemiluminescence, which allow for precise detection even at very low levels, facilitating the diagnosis of subclinical thyroid conditions.